Aqueous primer composition for enhanced film formation and method of using the same

ABSTRACT

A water-based bonding primer composition and a method of applying the same onto a metallic surface prior to adhesive bonding. The bonding primer composition is a water-based dispersion containing water, one or more epoxy resins, one or more curing agents, a silane compound, a low amount of propylene carbonate (PC), and optional additives. The bonding primer composition can form substantially smooth films by spraying, and at the same time, meet environmental regulations and provide high bonding performance.

This application claims the benefit of prior U.S. ProvisionalApplication No. 61/886,817, filed on Oct. 4, 2013, which is incorporatedherein in its entirety.

In the manufacture of composite structures, particularly in theaerospace and automotive industries, it is conventional to bond afabricated metallic structure to metallic or composite adherendsutilizing structural adhesives or to laminate one or more prepreg pliesof resin impregnated fibrous reinforcement to the fabricated metallicstructure. Bonding typically requires curing the structural adhesivesafter the structures are joined. In general, to ensure the greatestlevel of adhesive strength, the metal surface(s) are scrupulouslycleaned of dirt, soil, grease, and metal oxidation products immediatelyprior to bonding. Unfortunately, this procedure cannot be generally usedmost times as the cleaning and bonding operations are often separated bylong periods of inactive time. During such periods, the metal surfacemay become hydrolyzed, lessening the adhesive strength of the bond. Asolution to overcome this difficulty is to apply a primer on the cleanedmetal surface(s).

Conventional primers have often been prepared from thermosetting resinsdissolved in volatile organic solvents, e.g. acetone, isopropyl alcohol,tetrahydrofuran (THF), methyl ether ketone, ethylene glycol, xylene,toluene, ethyl acetate, and the like. Such solvent-based primers canform smooth films when sprayed onto metallic surfaces prior to curing.However, the use of primers containing large amounts of volatile organiccompounds (“VOCs”) is under increasing scrutiny for both toxicologicaland environmental reasons. Consequently, water-based primers arebecoming more desirable. However, commercially available water-basedbonding primers that contain epoxy resins tend to form a powdery(powder-like) coating when they are applied via spraying, and theresulting air-dried film has poor scratch or rub resistance before cure.This can be an issue in structural bonding if the primer is scraped-offbefore curing. Scratch or rub resistance is relating to the ability ofthe primer film, after being air-dried, to resist scratching or rubbingoff (i.e. being removed by rubbing) during the handling of the metallicpart with the primer film thereon.

Another significant consideration in the use of primers is thedurability and corrosion resistance of the joints formed between themetal surface and the material bonded to the metal surface. This isparticularly important in structural applications, such as aircraftstructures, because these joints are exposed to a wide range ofenvironmental conditions with extreme temperatures, high humidity, andhighly corrosive marine environments. To avoid the failure of the jointsas well as to meet stringent commercial passenger and cargo aircraftstandards, the adhesive-bonded joints of the structural components mustwithstand harsh environmental conditions, and, in particular, resistanceto corrosion and disbanding in humid, salt-laden environments,especially those resulting from sea spray or de-icing materials. Failureof these joints often starts with diffusion of water through theadhesive followed by corrosion of the underlying metal structure.

There remains a need for a method for bonding metallic structures usingprimer formulations that can form substantially smooth films byspraying, and at the same time, meet environmental regulations, providehigh bonding performance and corrosion protection.

SUMMARY

Disclosed herein is a method of applying a one part, water-based bondingprimer composition onto a metallic surface of a first substrate prior tobonding the metallic surface to a second substrate via a curableadhesive. The bonding primer composition is a water-based dispersioncontaining water, one or more epoxy resins, one or more curing agents, asilane compound, a low amount of propylene carbonate (PC), and optionaladditives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a photographic image showing the surface of a primer filmformed from a propylene carbonate—containing formulation according toone embodiment.

FIG. 1B is a photographic image showing the surface of a primer filmformed from a control formulation without solvent.

FIG. 2 is a photographic image showing the surface of a primer filmformed from a propylene carbonate—containing formulation according toanother embodiment.

DETAILED DESCRIPTION

The bonding primer composition of the present disclosure providescorrosion protection and enhanced bonding ability of metallic materials,particularly, aluminum and aluminum alloys used in the aerospaceindustry. It has been found that the presence of propylene carbonateenhances film formation of the primer composition by enabling theformation of a smooth primer film that is scratch-resistant andrub-resistant before cure and solvent-wipe resistant after cure. Inaddition, propylene carbonate can function as a buffering agent byreacting with certain water-soluble curing agents/catalysts in theprimer compositions to stabilize as well as provide a neutral pH (around7±0.5). Some water-soluble curing agents and catalysts, such asimidazole, tend to dissolve in the water-based primer composition toform a solution with a highly basic pH, thereby negatively affect theadhesion of the primer film to metal surface. As such, propylenecarbonate is a multifunctional component in the primer composition.

One aspect of the present disclosure is directed to a method of applyingthe water-based bonding primer composition onto a metallic surface of afirst substrate prior to bonding the metallic surface to a secondsubstrate. The bonding primer composition is a water-based (or aqueous)dispersion having a solid content from 10% to 25%, and containing:water, one or more thermosettable resins, curing agent(s)/catalyst(s), asilane compound, and a low amount of propylene carbonate (PC). Theamount of PC, in weight percentage, is less than 15 wt. % (weightpercent) based on the total weight of the composition, preferably,within the range of 1 wt. % to 10 wt. %. In some embodiments, the amountof PC may be about 10-15 g/L of water-based primer composition. Thebonding primer composition could be applied onto a metallic surface byspraying to form a smooth, continuous primer film. The term “smooth” inthis context refers to a film having an even surface that issubstantially free of perceptible projections, lumps, or indentations,and is not powder-like in appearance. Furthermore, the sprayed primerfilm is capable of being air-dried at ambient temperature (21° C.-26°C.) in 30 minutes or less, e.g. 15-30 minutes. Due to the low load levelof PC, the primer composition is readily in compliance with the currentOSHA (Occupational Safety and Health Administration) and REACH(Registration, Evaluation, Authorization and Restriction of Chemicals)requirements. Currently, propylene carbonate is considered a non-VOCchemical compound in the United States.

For adhesive bonding of a metallic substrate to another substrate (metalor composite substrate), the water-based primer composition of thepresent disclosure may be applied onto a metallic surface by spraying orbrushing to form a curable primer film. After being air dried at ambienttemperature for less than 30 minutes, the curable primer film isresistant to scratching and rubbing off. The metallic surface ispreferably pre-treated prior to applying the primer composition in orderto enhance the adhesion of the metallic surface to the subsequentlyapplied primer film and to provide corrosion resistance to the metalsurface. The primer film is cured in an oven at an elevated temperature(e.g. 250T or 350T for one hour) prior to bonding assembly. The primedsurface of the metallic substrate is then adhered to the secondsubstrate by providing a curable, polymeric adhesive film between theprimed surface and the second substrate. The second substrate may beanother metallic substrate or a composite substrate composed ofreinforcement fibers embedded in or impregnated with a matrix resin. Theadhesive may be applied onto a surface of the second substrate, oralternatively, the adhesive may be applied onto the primed surface ofthe first substrate. The resulting assembly is then subjected to curingat an elevated temperature to cure the adhesive, and consequently, toproduce a bonded structure. Curing may be carried out by applying heatand pressure to the assembly. The primer composition is formulated sothat it can be compatible with conventional curable, polymeric adhesives(particularly, epoxy-based adhesives) that are curable at temperatureswithin the range of 250° F. to 350° F. (121° C. to 177° C.).

The term “substrate” as used herein includes layers and structures ofany shape and configuration.

The terms “cure” and “curing” as used herein refer to the hardening of apolymer material by cross-linking of polymer chains, brought about bychemical additives, ultraviolet radiation or heat. Materials that are“curable” are those capable of being cured, i.e. becoming harden.

When the second substrate is a composite substrate composed ofreinforcement fibers and matrix resin, the matrix resin may be partiallyor fully cured, or uncured. When the matrix resin is uncured or onlypartially cured prior to the adhesive joining of the two substrates,full curing of the matrix resin occurs simultaneously with the curing ofthe adhesive during the bonding stage.

The primer composition may be applied (e.g. by spraying) to the metallicsurface in several layers until a desired film thickness is achieved.For example, the amount of primer composition is applied so that thecured primer film may have a thickness from about 0.0001 inch to about0.0003 inch (or 0.1 mil to 0.3 mil).

The water-based primer composition disclosed herein preferably includesinorganic or organic corrosion inhibitors to further improve long-term,anti-corrosion performance.

To enhance the adhesion of the metallic surface to the subsequentlyapplied polymeric primer film, the metallic surface may be pre-treatedprior to applying the primer compositions thereon. Suitable surfacetreatments include wet etching, anodization such as phosphoric acidanodization (PAA), and phosphoric acid/sulfuric acid anodization (PSA),and sol-gel processes that are known to those skilled in the art. A morespecific example of a suitable surface treatment is ASTM D2651, whichincludes cleaning with a soap solution, followed by wet etching and thenanodizing with an acid. The water-based primer composition disclosedherein is formulated to be compatible with these various surfacetreatments.

PAA typically involves using phosphoric acid (e.g. ASTM D3933) to formmetal oxide surfaces, and PSA typically involves usingphosphoric-sulphuric acid to form metal oxide surfaces. Anodizingproduces a porous, rough surface into which the primer composition canpenetrate. Adhesion results primarily from mechanical interlockingbetween the rough surface and the primer film.

Sol-gel process typically involves the growth of metal-oxo polymersthrough hydrolysis and condensation reactions of an aqueous solution oforgano-functional silane and zirconium alkoxide precursors to forminorganic polymer networks on the metal surface. The sol-gel coating canprovide good adhesion between the metal surface and thesubsequently-applied primer film via covalent chemical bonding.

Thermosettable Resins

The preferred thermosettable resins are epoxy resins. Suitable epoxyresins include multifunctional epoxy resins having functionality of atleast about 1.8, or at least about 2 functionalities. The epoxy resinsare optionally chain-extended, solid glycidyl ethers of phenols, such asresorcinol and the bisphenols, e.g., bisphenol A, bisphenol F, and thelike. Also suitable are the solid glycidyl derivatives of aromaticamines and aminophenols, such asN,N,N′,N′-tetraglycidyl-4,4′diaminodiphenylmethane. Moreover, the epoxyresins may have an epoxy equivalent weight (EEW) of about 145-5000, withan equivalent weight of about 300-750 being preferred, and an equivalentweight of 325 being most preferred.

The epoxy resins may be in solid form, or a dispersion of solid epoxy.The epoxy resin in dispersed phase may be a dispersion of more than oneepoxy resin in the form of a mixture of distinct particles, or mayconsist of only one type of particles containing more than one epoxyresin per particle. Thus, a flexibilizing epoxy such as the highermolecular weight bisphenol A or bisphenol F epoxies may be blended witha high-temperature resistant epoxy such as tetraglycidyl methylenedianiline (TGMDA), then the mixture is cooled, ground, or otherwisedispersed into solid particles of the requisite size. These same epoxyresins might be advantageously dispersed separately without blending.

A mixture of different epoxy resins may be used. In one embodiment, themixture of epoxy resins includes novolac epoxy resin and diglycidylether of bisphenol A (“DGEBA”) resin. Examples include novolac epoxyresins such as Epirez 5003 available from Huntsman, and bisphenol Aepoxy resins such as XU-3903 available from Huntsman and D.E.R. 669available from Dow Chemical Co. In another embodiment, the resin mixturecontains an epoxy resin having a functionality of about 4 or less, andan epoxy resin having a functionality of about 5 or more. The use ofhigher functionality epoxy resins, i.e., epoxy resins having afunctionality of five or more, in minor amounts is suitable, forexamples less than 40 wt. % based on the sum of the weights of all epoxyresins in the composition. The use of such higher functionality epoxyresins in such minor amounts has been found to increase the solventresistance of the cured primer composition without substantiallylowering the adhesive properties.

In one embodiment, the primer composition includes a mixture of thefollowing epoxy resins:

1) from 30 to 70 wt. % of an epoxy resin having a functionality of fromabout 1.8 to about 4 and an epoxy equivalent weight of from about 400 toabout 800;

2) from 5 to 20 wt. % of an epoxy resin having a functionality of fromabout 1.8 to about 4 and an epoxy equivalent weight of from about 2000to about 8000; and

3) from 10 to 40 wt. % of an epoxy resin having a functionality of about5 or more and having an epoxy equivalent weight of from about 100 toabout 400,

wherein the weight percentages totaling 100% based on total weight ofthe epoxy mixture.

The total amount of epoxy resin(s) may be about 20-60% by weight basedon total weight of the primer composition.

Curing Agents and Catalysts

The water-based primer composition contains one or more curing agentsand/or catalysts which may be water-soluble or water-insoluble. Suitablecuring agents include a water-soluble, substituted amino triazine suchas 2-β-(2′-methylimidazolyl-1′1-ethyl-4,5-diamino-s-triazine (which iscommercially available as CUREZOL 2 MZ-Azine®); a modified polyamine,e.g. Ancamine 2014®; dicyandiamide (DICY), or a water-insoluble curingagent such as a bisurea based curing agent (such as Omicure 24 from CVCChemicals) or Toluene-2,4-bis(N,N′ dimethyl urea) (such as Omicure U-24from CVC Chemicals); amine-epoxy adducts and/or an aromatic amine suchas bis(3-aminopropyl)-piperazine (BAPP) (available from BASF).

Catalysts may be added as an optional component to speed up thecuring/crosslinking of the thermoset resins or to enable curing at lowertemperatures. Solid, water-dispersible catalysts may be added when aparticular curing agent is not sufficiently active at the primercomposition's heating temperature to effect cure of the primercomposition. For example, where a curing agent is active at 350° F., acatalyst is added to enable curing at around 250° F. The catalyst may bewater soluble or water insoluble, and may be in particulate form havinga particle size such that essentially 100 percent of the particles havea mean diameter of less than about 30 μm. The mean diameter of theparticles can be measured by laser light diffraction method usinginstruments such as Malvern Mastersizer 2000 and Horiba LA-910. Typicalcatalysts that may be employed include, but are not limited to:bisureas, blocked imidazoles, substituted imidazoles or other blockedamines such as amine/epoxy adducts, hydrazines, etc.

The curing agent(s), solely or in combination with one or morecatalyst(s), may be present in amounts from about 2 to 30 parts per 100parts of the epoxy resin in total (i.e. total amount of epoxy orepoxies).

Silane Compounds

The silane compound in the water-based primer composition has silanefunctional groups that can react or bond to the material to be bonded toa metallic surface. Suitable silane compounds include organosilanes.Organosilanes having hydrolyzable groups are preferred. In certainembodiments, the organsilanes have the following general formula:

wherein n is greater than or equal to 0; wherein each X is OH, OCH₃, andOCH₂H₅; wherein R₁ is CH═CH₂,

or CH₂—CH₂—CH₂—Y, wherein Y is NH₂, SH, OH, NCO, NH—CO—NH₂,NH—(CH₂)₃NH₂, NH-Aryl,

and wherein each R₂ is alkyl, alkoxy, aryl, substituted aryl, or R₁.

Examples of suitable, commercially available organosilane compounds arethose available from OSi Specialties Inc., Danbury, Conn. including, butare not limited to, A-186, a beta-(3,4-epoxycyclo hexyl)ethyltrimethoxysilane; A-187, a gamma-glycidoxypropyltrimethoxysilane; A-189, agamma-mercaptopropyltrimethoxysilane; A-1100, agamma-aminopropyltriethoxysilane; A-1106, an aminoalkyl siliconesolution; A-1170, a bis-(gamma-trimethoxy-silylpropyl)amine; Y-9669, aN-phenyl-gamma-aminopropyl-trimethoxysilane; Y-11777, an amino alkylsilicone/water solution; and Y-11870, an epoxy functional silanesolution. Other suitable commercially available organosilanes include,but are not limited to, Z-6040, a gamma-glycidoxypropyl-trimethoxysilane from Dow Corning, Midland, Mich., HS2759, an aqueous epoxyfunctional silane; HS2775, an aqueous amino silane solution; and HS2781an aqueous oligomeric silane solution with amino and vinyl groups allsold by Huls America Inc., N.J. Another example is3-glycidoxypropylmethoxysilane, which is sold under the trademarkZ-6040.

Generally, the organosilane is present in the water-based primercomposition in amounts ranging from about 0.01 to 15 parts per 100 partsof water, preferably from about from about 0.1 to 10 parts per 100 partsof water.

The organosilane may be in a liquid or powder form that can be addeddirectly to the water-based primer composition.

Corrosion Inhibitors

Chromates or non-chromate corrosion inhibitors may be used in thewater-based primer composition disclosed herein, however, to comply withenvironmental, health and safety regulations, non-chromate compounds arepreferred. Examples of suitable chromate corrosion inhibitors includestrontium chromate, barium chromate, zinc chromate, and calciumchromate. Non-chromate corrosion inhibitors include inorganic compoundscontaining one or more ions selected from the group consisting of NaVO₃,VO₄, V₂O₇, phosphate, phosphonate, molybdate, cerium, and borate.Examples of inorganic, non-chromate corrosion inhibitors include, butare not limited to, a metavanadate anion, such as sodium metavanadate, acombination of a molybdate and metavanadate, or any combinations ofmolybdate, metavanadate, phosphate, phosphonate, cerium and borate. Alsosuitable are organic corrosion inhibitors, including those that arechemically anchored to the surface of a particle or encapsulated and arereleasable in the event of corrosion. Examples of such releasableorganic corrosion inhibitors are described in U.S. Patent ApplicationPublication 2010/0247922, published on Sep. 30, 2010. A combination ofdifferent corrosion inhibitors may be used.

The total amount of corrosion inhibitor may be within the range of 1-7wt. % based on the total weight of the primer composition.

Optional Additives

The water-based primer composition may optionally contain conventionaldyes, pigments, and inorganic fillers. The total amount of such optionaladditives is less than 3 wt. %, for example, 0.1 wt. % to 2 wt. %. Abenefit of compositions containing dyes or pigments is that the surfacecoverage can be assessed more easily by visual methods. Inorganicfillers, in particulate form, are added in order to control the rheologyfor application process and stability. Suitable inorganic fillersinclude fumed silica, clay particles, and the like.

According to one embodiment, the water-based primer composition is aspray-able dispersion having a pH of 6-8 and containing:

-   -   (i) 20-60 wt. % one or more epoxy resin(s);    -   (ii) 2-30 parts curing agent(s), solely or in combination with        catalyst(s), per 100 parts of epoxy resin(s) in total;    -   (iii) organosilane in an amount of 0.1 to 10 parts per 100 parts        of water;    -   (iv) 1-10 wt. % propylene carbonate;    -   (v) 1-7 wt. % at least one chromate or non-chromate corrosion        inhibitor;    -   (vi) optionally, 0.1-2 wt. % inorganic fillers in particulate        form and/or pigment/dyes;    -   (vii) water to provide 10%-25% solids,

where “wt %” represents weight percentage based on the total weight ofthe composition.

EXAMPLES

The following examples show the performance results obtained using awater-based bonding primer formulation with a low amount of propylenecarbonate as compared to other primer formulations which do not containpropylene carbonate.

Example 1

Primer formulations were prepared according to the formulation disclosedin Table 1.

TABLE 1 Formulation 1 Formulation 2 Components Amount Amount Bisphenol Aepoxy (dispersion with 83 gms 83 gms 55% solids) Solid epoxy novolacresin 11 gms 11 gms Solid Bisphenol A-based epoxy 14 gm  14 gm  (powder)2,2-Bis-4-(4-aminophenoxy) phenyl 10 gms 10 gms propane (BAPP)Toluene-2,4-bis (N,N′-dimethyl urea)  3 gms  3 gms Paliotol Yellow(pigment) 0.3 gms  0.3 gms  Strontium chromate (corrosion 15 gms 15 gmsinhibitor) Fumed silica  2 gms  2 gms Organosilane (gamma- 1 wt % oftotal 1 wt % of total glycidoxypropyltrimethoxy silane) water amountwater amount Mergal K10N (biocide) 0.1 wt % of 0.1 wt % of compositioncomposition Propylene carbonate 25 gms Propylene glycol 25 gms Deionized(DI) water To provide To provide 20 wt % solids 20 wt % solids

The pH of the primer formulations in Table 1 was approximately 6.5.

Each of the primer formulations was sprayed onto surface treated AI-2024alloy using HVLP (high velocity low pressure) gun to form a film having0.2 mil thickness. The surface treatment was according to ASTM D 2651,which includes cleaning, FPL etching and PAA anodization. The resultinguncured films were left to dry by air at ambient temperature. Forcomparison, a Control primer film was formed by the same method using aformulation without solvent—Formulation 1 without PC.

Evaluation of the primer films was carried out and the results are shownin Table 2.

TABLE 2 Control (No solvent) Formulation 1 Formulation 2 Dry filmappearance Powdery Smooth and Smooth and adherent adherent Film dryingtime Pass Pass Not pass Scratch resistance Not pass Pass Pass beforecure

For the film drying time, “pass” means the film was dried by air atambient temperature in less than 30 minutes after spraying, and “notpass” means it took too long to dry after spraying, or more than 30minutes.

Scratch resistance test involved rubbing the air-dried, uncured filmwith fingers or using a dry, white cloth to rub the film surface to seeif any film material became attached to the cloth after rubbing severaltimes.

The surface of the air-dried, uncured primer film formed fromFormulation 1 was found to be smooth and scratch resistant, and is shownin FIG. 1A (a photographic image of the primer surface). In contrast,the air-dried, uncured Control film had a powdery appearance as shown inFIG. 1B, and did not pass the scratch resistance test. The uncuredprimer film formed from Formulation 2 took too long to dry at ambienttemperature after spraying, more than 30 minutes, and did not pass thescratch resistance test.

Example 2

Primer formulations were prepared according to the formulation disclosedin Tables 3, 4, and 5.

TABLE 3 Formulation 3 Formulation 4 Components Amount Amount Bisphenol Aepoxy (dispersion in 133 gms  133 gms  water with 55% solids) Epoxynovolac resin (dispersion 72 gms 72 gms with 54% solids) Solid BisphenolA-based epoxy  9 gms  9 gms (powder) Formulated amine based curing 11gms 11 gms agent 2-β-(2′-methylimidazolyl-1′1-ethyl-  4 gms  4 gms4,5-diamino-s-triazine Fumed silica 0.3 gms  0.3 gms  Dicyandiamide(DICY)  3 gms  3 gms Paliotol Yellow (pigment) 1.8 gms  1.8 gms  Zincphosphate-based corrosion 24 gms 24 gms inhibitor Organosilane(gamma-glycidoxy 1 wt % of total 1 wt % of total propyltrimethoxysilane) water amount water amount Mergal K10N (biocide) 0.1 wt % of 0.1wt % of composition composition Propylene carbonate 20 gms Propyleneglycol 20 gms DI water To provide To provide 20 wt % solids 20 wt %solids

TABLE 4 Formulation 5 Formulation 6 Components Amount Amount Bisphenol Aepoxy (dispersion in 133 gms  133 gms  water with 55% solids) Epoxynovolac resin (dispersion in 72 gms 72 gms water with 54% solids) SolidBisphenol A-based epoxy  9 gms  9 gms (powder) Formulated amine-basedcuring 11 gms 11 gms agent 2-β-(2′-methylimidazolyl-1′1-ethyl-  4 gms  4gms 4,5-diamino-s-triazine Fumed silica 0.3 gms  0.3 gms  Dicyandiamide(DICY)  3 gms  3 gms Paliotol Yellow (pigment) 1.8 gms  1.8 gms  Zincphosphate-based corrosion 24 gms 24 gms inhibitor Organosilane(gamma-glycidoxy 1 wt % of total 1 wt % of total propyltrimethoxysilane) water amount water amount Mergal K10N (biocide) 0.1 wt % of 0.1wt % of composition composition Acetone 28 gms 2-Propoxyethanol 28 gmsDI water To provide To provide 20 wt % solids 20 wt % solids

TABLE 5 Formulation 7 Formulation 8 Components Amount Amount Bisphenol Aepoxy (dispersion in 133 gms  133 gms  water with 55% solids) Epoxynovolac resin (dispersion in 72 gms 72 gms water with 54% solids) SolidBisphenol A-based epoxy  9 gms  9 gms (powder) Formulated amine-basedcuring 11 gms 11 gms agent 2-β-(2′-methylimidazolyl-1′1-ethyl-  4 gms  4gms 4,5-diamino-s-triazine Fumed silica 0.3 gms  0.3 gms  DICY  3 gms  3gms Paliotol Yellow (pigment) 1.8 gms  1.8 gms  Zinc phosphate-basedcorrosion 24 gms 24 gms Inhibitor Organosilane (gamma-glycidoxy 1 wt %of total 1 wt % of total propyltrimethoxy silane) water amount wateramount Mergal K10N (biocide) 0.1 wt % of 0.1 wt % of compositioncomposition Acetone 14 gms  7 gms 2-Propoxyethanol 14 gms 17 gms2-Propanol 17 gms DI water To provide To provide 20 wt % solids 20 wt %solids

Each of the primer formulations was sprayed onto surface treated AI-2024alloy using HVLP gun to form a film having 0.2 mil thickness asdiscussed in Example 1. For comparison, a Control primer film was formedby spraying a primer formulation without any solvent—Formulation 3without propylene carbonate.

The air-dried, uncured primer films were evaluated and the results areshown in Table 6. The pH of the primer formulations are also shown inTable 6. Additionally, Single Lap Shear test (ASTM D1002) and FloatingRoller Peel (ASTM D3167) test were carried out to determine the bondingperformance of the primer films. These tests were done after bonding theprimed surface to another aluminum alloy (AI 2024T3) sheet using acurable, epoxy-based adhesive (FM 73M from Cytec Industries Inc.),followed by curing. Table 6 shows the results of the mechanical tests.

TABLE 6 Scratch Solvent wiping Dry film Film resistance resistanceFailure mode of appearance drying time before cure pH after curemechanical test Control Powdery Pass Not pass 9.2 Not pass SometimesPrimer (No solvent) failure observed Formulation 3 Smooth and Pass Pass7.0 Pass Cohesive failure adherent Formulation 4 Smooth and Not passPass 9.0 Pass — adherent Formulation 5 Smooth and Pass Pass 9.0 Not passPrimer failure adherent Formulation 6 Smooth and Pass Pass 9.0 Not passPrimer failure adherent Formulation 7 Smooth and Pass Pass 9.0 Not passPrimer failure adherent Formulation 8 Smooth and Pass Pass 9.0 Not passPrimer failure adherent

The air-dried, uncured primer film formed from Formulation 3 washomogeneous and smooth, and exhibited good scratch resistance. FIG. 2shows the surface of the primer film formed from Formulation 3. Incontrast, the uncured Control film was powdery in appearance and hadpoor scratch resistance. Moreover, the pH value of the Controlformulation (without propylene carbonate) was about 9.2 due to thepresence of the water-soluble catalyst2-β-(2′-methylimidazolyl-1′1-ethyl-4,5-diamino-s-triazine. With theaddition of propylene carbonate, the pH of Formulation 3 was effectivelylowered to around 7, exactly at the desired neutral level. Here,propylene carbonate acts not only as a film promoter but also as abuffer agent to bring the pH of the primer composition from basic toaround 7 by reacting with the water-soluble catalyst.

Although Formulations 4-8 were capable of forming smooth films byspraying, these films failed to provide the bonding performance desireddue in part to the high pH. Moreover, Formulation 4 took too long todry. It has been found that the neutral pH of the water-based primer canoptimize surface adhesion via the silane coupling agent and is normallyrequired for overall balanced performance of the water-based primer.

What is claimed is:
 1. A surface treatment method for treating ametallic surface prior to adhesive bonding comprising: applying awater-based primer composition onto a metallic surface to form a curableprimer film having a smooth, continuous surface, said water-based primercomposition comprising: one or more epoxy resins; at least one curingagent; a silane compound having at least one hydrolyzable group;propylene carbonate; and water, wherein the water-based primercomposition has a solid content of 10%-25%, and the amount of propylenecarbonate is less than 15% by weight based on the total weight of theprimer composition.
 2. The surface treatment method of claim 1, whereinthe water-based primer composition is applied by spraying.
 3. Thesurface treatment method of claim 1, wherein the pH of the primercomposition is within the range of 6-8.
 4. The surface treatment methodaccording to claim 1, wherein the curing agent is water-soluble, andwithout propylene carbonate, the pH of the primer composition would behigher.
 5. The surface treatment method according to claim 1, whereinthe primer film is air-dried in 30 minutes or less at ambienttemperature (21° C.-26° C.).
 6. The surface treatment method accordingto claim 1, wherein the water-based primer composition furthercomprising a corrosion-inhibiting compound.
 7. The surface treatmentmethod according to claim 1, wherein the metallic surface is the surfaceof an aluminum or aluminum alloy substrate.
 8. The surface treatmentmethod according to claim 1, wherein the metallic surface is subjectedto anodization or sol-gel process to form a metal oxide coating prior toapplying the water-based primer composition.
 9. A bonding methodcomprising: (a) applying a water-based primer composition onto a surfaceof a metallic substrate to form a curable primer film having a smooth,continuous surface, said water-based primer composition comprising: oneor more epoxy resins; at least one curing agent; a silane compoundhaving at least one hydrolyzable group; propylene carbonate; and water,wherein the water-based primer composition has a solid content of10%-25%, and the amount of propylene carbonate is less than 15% byweight based on the total weight of the primer composition; (b)adhesively joining the metallic substrate to a second substrate wherebya curable, polymeric adhesive is positioned between the primer film andthe second substrate; and (c) curing the adhesive to form a bondedstructure.
 10. The bonding method of claim 9, wherein the water-basedprimer composition is applied at step (a) by spraying.
 11. The bondingmethod of claim 9, wherein the water-based primer composition furthercomprising a corrosion-inhibiting compound.
 12. The bonding methodaccording to claim 9, wherein the second substrate is another metallicsubstrate.
 13. The bonding method according to claim 9, wherein thesecond substrate is formed of aluminum or aluminum alloy.
 14. Thebonding method according to claim 9, wherein the second substrate is acomposite substrate comprising a resin matrix and reinforcement fibers.15. The bonding method of claim 14, wherein the composite substrate isuncured when it is being joined to the metallic substrate at step (b).16. The bonding method of claim 14, wherein the composite substrate ispartially or fully cured when it is being joined to the metallicsubstrate at step (b).